The Keystone Receptacle uses the same formula as the spell Recall to determine essence cost over teleport distance.
If insufficient essence is in the gateway to take you all the way to your destination, you will be deposited somewhere in the middle, as far as the essence can take you.

A keystone receptacle does not have to have a keystone in it. When it does not, it is referred to as on the "Open" gateway network. Anyone may arrive at an open gateway.
When a keystone is added to the receptacle, only players who activate the gateway system using the matching keystone will be allowed to arrive. In this manner, many separate gateway networks can be established.

To activate a Keystone Receptacle, simply right click it. If it is built into the proper structure, the gateway will activate and the portal will open. Step through, and arrive at your destination.
By default, it will activate on the Open network. To activate it on a specific network, activate it with that network's matching keystone equipped as your active item.